An important process in the fabrication of integrated circuits is the removal of various layers of materials formed on the silicon wafer. Such removal must be highly selective. First the removal of material is accomplished on a wafer containing a patterned coating of, for example, photoresist which permits removal of material only from selected areas of the wafer. At the same time the amount of material removed is critical. For example, only a particular layer of material should be removed without disturbing the underlying layer or substrate. Thus, it is important to know or detect when all of the selected material has been removed so that the process can be stopped before the substrate is damaged.
Several etching techniques are now in common usage. One such technique utilizes liquid chemicals into which a photoresist pattern silicon wafer is immersed in a chemical solution. The chemical attacks the material not covered by the photoresist. This method has the disadvantage of being isotropic, i.e., the edges on the etched surfaces are not well defined due to undercutting which is the etching away of the walls or that portion of the material covered on its surface by the photoresist.
With the demand for smaller geometries and line widths on integrated circuits a technique known as plasma or dry etching has been evolved. This method is anisotropic and eliminates undesirable undercutting. In this method a gas such as CF.sub.4 is injected into a chamber containing the wafer to be etched. The chamber is maintained at a relative vacuum and the gas is turned into a plasma by coupling the chamber to an R.F. frequency power source. This creates radicals which are chemically reactive with the surface to be etched, thus, removing the desired material which is continuously removed from the chamber. Like all methods it is important to detect when the desired material is removed.